This invention relates generally to method and apparatus for controlling an internal combustion engine in accordance with various information including atmospheric pressure.
As a system for controlling an internal combustion engine, such as motor vehicle engines, is known a speed density control system (referred to as S-D system) in which necessary amount of fuel is calculated using two parameters indicative of the intake air pressure in the intake manifold and the rotational speed of the engine crankshaft.
When an internal combustion engine is controlled with such a conventional S-D system without effecting a feedback control, the air/fuel ratio of the air/fuel mixture supplied to the engine is apt to be deviated toward the lean side when atmospheric pressure (Patm) decreases when a motor vehicle is driven at a high altitude because it cannot be considered that exhaust manifold pressure (Pr) lowers and volumetric efficiency (.eta.v) increases when the amount of fuel is determined on the basis of only the intake manifold pressure (Pm) and engine speed (N). This fact is indicated by the following formula (1), and as a result of such deviation in air/fuel ratio, the drivability and engine starting condition become poor. EQU .eta.v.varies.(Pm/Patm)X[1+(1-Pr/Pm)/(K.sup.(.epsilon.-1) ](1)
wherein
K is a constant; and PA1 .epsilon. is a compression ratio
In order to compensate for such undesirable deviation of the air/fuel ratio toward the lean side, an atmospheric pressure sensor or an exhaust gas pressure sensor has been employed independent of the intake manifold pressure sensor. However, such an engine control system suffers from increase in cost inasmuch as it requires two separate pressure sensors, while high reliability is not insured because of variations in accuracy between two separate sensors.